It is recognised that the sewage flow from a main collection sewer system consists primarily of water containing organic solids which are to be removed from the water by the sewage treatment plant prior to discharge of the cleaned water back to the environment. However, additional to the organic load the sewage flow includes wood, paper, cotton, rags and the like (usually referred to as “screenings”) together with grit and stones. It is usual to pass the raw sewage flow through screens or other filtering devices which extract paper rags and the like together with large stones; allowing the sewage flow with entrained grit and small stones to pass further into the sewage treatment plant.
Down-stream of the screens it is usual to pass the sewage flow through one or more grit traps which extract the grit and small stones from the flow leaving the water with entrained organic solids to pass into the sewage treatment plant for biological treatment. It will be recognised however that in some treatment plants the screening takes place downstream of the grit trap(s) and so it is desirable that the grit trap(s) are able to accommodate screenings in the flow.
Conventionally grit traps can take a number of forms, but a particularly convenient form is a vortex grit trap in which the sewage flow with entrained grit and small stones enters a generally circular-cylindrical separation chamber tangentially. Positioned below the separation chamber, and coaxial therewith is a smaller diameter circular-cylindrical grit collection chamber, and usually but not exclusively the interface of the settlement chamber and the grit collection chamber is a frusto-conical region which guides grit falling from the flow in the settlement chamber into the collection chamber.
Periodically grit collecting in the collection chamber is removed for disposal by means of an air-lift or other convenient extraction mechanism.
As mentioned above the inlet flow is directed tangentially into the settlement chamber of the grit trap and thus forms a rotating flow within the chamber. The intention is that liquid with entrained organic solids will exit from the chamber over an outlet weir at substantially the same rate that inlet flow enters the chamber. The rotating flow within the chamber defines a relatively low speed vortex which provides an opportunity for the heavier grit and stones to separate under gravity from the organic solids which are of lower density and also tends to cause the settlings solids, both grit and organic solids, to migrate towards the axis of the vortex, so as to collect in the collection chamber of the grit trap. The nature of the vortex flow in the separation chamber of grit trap is a compromise between grit settlement and separation of organic solids. It will be recognised that if the rotational speed and flow through the chamber is too slow then organic solids will settle with the grit. On the other hand if the speed is too high then the finer grit will not settle and will remain suspended in the flow exiting the grit trap. The speed and volume of the inlet flow to the grit trap is not constant and so in order to provide greater constancy of the conditions in the trap there is provided a rotating impeller which ensures a predetermined rotating rate of flow in the separation chamber of the trap. Usually the compromise which is selected is flow conditions which permit settlement of substantially the whole of the stone and grit load of the inlet flow, and inevitably therefore some organic solids settle with the grit either by gravity separation, or by being attached to settling grit particles. Accordingly grit collecting in the collection chamber of the grit trap is contaminated to some extent with organic solids. It is known to try to improve the separation of organic solids from grit by modifying the rotating impeller so that the vortex flow generated has an upward component. It is highly desirable that collected grit for disposal is in a sufficiently clean condition for disposal without further treatment, but the presence of organic solids with the settled grit extracted from the grit trap often necessitates washing of the grit extracted from the grit trap prior to disposal.
The problem of contamination of the settled grit with organic solids has been recognised. U.S. Pat. No. 3,941,698 discusses the problem, and suggests the use of an “air scour” whereby periodically air is pumped into the grit collection chamber through four radially disposed pipes with the objective that the air flow will dislodge organic materials from the already settled grit. Clearly however the application of air is periodic, and is localised within the grit collection chamber so that a substantial amount of the collected grit will be untreated by the air scour.
The use of an air scour is also disclosed in our earlier British Patent 2148744 where a single pipe 22 extending axially of the grit collection chamber, and which normally forms part of the air lift for removing grit from the chamber, can be used periodically in reverse to pump air into the grit collection chamber again with the objective of dislodging organic solids from the already settled grit and floating it upwardly through the collection chamber and the separation chamber. However, it is apparent that the pipe 22 can only treat grit in the immediate vicinity of the pipe outlet.
British Patent 830531 discloses an arrangement where air is bubbled into the body of liquid in the separation chamber. It should be noted however that the disclosure in British Patent 830531 is of an aeration system and not an “air washing” arrangement. The objective of aeration is to ensure that there is sufficient oxygen in the liquid sewage to ensure subsequent biological processing of the organic load. Aeration as disclosed in British Patent 830531 would have little or no effect on the entrapment of organic solids with the settled grit.
It is an object of the present invention to provide a grit trap affording efficient settlement of grits while minimising the amount of organic solids retained with the grits.